Coal reserves in the U.S. are sufficient to meet anticipated transportation fuel needs for the foreseeable future. However, despite increased costs and security concerns associated with relying on imported petroleum, no company has built a U.S. facility to produce transportation fuel from coal.
Coal liquefaction methods, also known as coal-to-liquid (CTL) methods, refer to the process of converting coal into hydrocarbon liquids. The hydrocarbon products are typically used as transportation fuels, and include diesel fuels and jet fuels. For some CTL methods, coal gasification is a companion process that plays an important initial role by converting the coal to syngas, which is a mixture of H2 and CO. Typically, coal gasification is carried out in a gasification reactor into which coal is injected either by a gas-blown system or water slurry. Gas-blown systems typically use air to blow the coal into the hot zone in the gasifier. The syngas product is converted to transportation fuels via one of several methods (for example, in the Fischer-Tropsch process) depending on a variety of factors.
Current CTL methods emit substantial amounts of CO2 and consume large quantities of water. The insufficient hydrogen content of coal (H/C˜0.4-0.8) relative to transportation fuels such as JP-8 (H/C˜1.9, where for simplicity JP-8 is assumed to have the average formula C11H21) means that any process that purports to convert all, or substantially all, of the carbon in coal into carbon in JP-8 requires an additional source of hydrogen. To make transportation fuel, current CTL plants typically produce additional hydrogen in a post-gasification unit using the water-gas shift (WGS) process that consumes water and CO to produce H2 and CO2. The CO2 emissions increase further when plants convert H2 and CO to CO2 and hydrocarbons using Fischer-Tropsch synthesis (FTS), which produces a broad distribution of hydrocarbons and requires additional investment to convert fractions outside of the diesel range to diesel.
Current CTL technologies are not carbon-efficient. In a conventional indirect approach for coal to transportation fuels through FTS, as much as ⅔ of the carbon in the coal is emitted as CO2 (a 100,000 BPD plant would produce 136 million lb of CO2 per day). Because of these expensive process units, current CTL plants require˜$80,000 in capital per barrel/day of capacity.
There remains a need for converting coal to liquids at a competitive price on a large scale (e.g., 100,000+ barrel-per-day) in a method that produces little or no CO2 emissions and consumes little or no water. Preferred methods would be capable of using a variety of carbonaceous inputs and would be readily scalable to any desired level of output.